The Theme of Health in Genetics

Unfortunately, not everyone can stomach a 3 scoop sundae covered in
chocolate fudge and whipped cream - some get some rather discomforting
digestive issues we don't want to describe in detail here. This is very
often the consequence of lactose intolerance: the inability to break
down lactose, a common sugar in milk and milk-derived products. Lactose
intolerance in adults is often viewed as a disease, yet this condition
is the norm not only among humans, but all mammals.

The lactase
enzyme in intestinal cells allows young mammals to digest milk. After
weaning, however, its expression decreases dramatically as lactose is no
longer an essential part of a mammal's diet. Humans, on the other hand,
are rather unique mammals: some of us like milk with our breakfast
cereal and in our coffee even as adults. Most humans are lactose
intolerant to some degree in adulthood (in fancy terms, this condition
is known as "lactase nonpersistance" or "adult hypolactasia"). As we've
already said, this is because the body naturally makes less and less
lactase as you get older because milk no longer forms the major part of
your diet (unlike when you were a baby) and its wasteful in terms of
resources to keep on making the same amount as when you were dependent
on milk for all of your nutrition. But in some people this happens at a
very young age, sometimes as early as two or three years old; this type
of early lactose intolerance is genetically inherited. It's even
possible for newborn babies to be lactose intolerant due to other
inherited gene mutations, meaning that there's either little or no
lactase, or the lactase produced (in normal quantities) just doesn't
work. And, as you can imagine, if you can get some forms that don't work
at all, then you can also get some forms that work even better than
average, so some people's lactase is just more effective than others,
even if there's only a small amount of it being made. As ever,
environment can have an effect, too: some ladies who are lactose
intolerant regain their ability to happily process dairy products whilst
they're pregnant! Some drug treatments, such as chemotherapy and
antibiotics, can also cause lactose intolerance, as can some illnesses,
like Crohn's disease.

In some Northern European, African and
Middle Eastern populations and their descendants, lactase activity
persists into adulthood thus making them lactose tolerant. This
condition is genetically determined, with lactose tolerance being the
dominant trait. The mutations that gave rise to this new phenotype
evolved recently in evolutionary terms; research suggests it only
appeared in Europe within the last 8000 years (Burger, et al., 2007).
Also, lactose tolerance has evolved in humans more than once: in other
words, the same phenotype arose in different populations independently -
the mutations that allow Northern Europeans to digest lactose in
adulthood are different than those that let East African or Saudi
communities enjoy milk (Enattah, et al., 2008; Tishkoff, et al., 2007).
Lactose tolerance in all these different groups most likely evolved
through natural selection because of the many adaptive benefits of milk
consumption. Thus losing the ability to produce lactase as you get older
is considered the "wild phenotype," while persistent lactose tolerance,
the rare and more recently evolved trait, is the "mutant" phenotype.